Excimer lasers operating at 193 nm and 248 nm are increasingly used as light sources for mask exposure in semiconductor manufacturing to decrease the feature size of integrated circuits. This allows more circuit elements per circuit area and increases wafer throughput in the factory thus reducing overall costs. To further enhance the rendition of the mask features, the lasers line is narrowed to produce very narrow spectral ranges, typically 10 picometers or less. Diffraction gratings, in particular echelle gratings are used to this end. The disadvantage of the echelle gratings is their low damage threshold to laser radiation which limits their useful service life, and their low diffraction efficiency at steep angles of incidence (in the order of 78°), resulting in substantial replacement costs.
Traditional gratings employed for wavelength-selective feedback in laser cavities are generally replica gratings, where the groove structure is an epoxy type of material overcoated with a metal (typically aluminum for operation in the ultraviolet), and possible overcoated with a thin layer of a dielectric (typically magnesium dioxide) to suppress aluminum oxide buildup. The purpose of the dielectric overcoat in this case is to help prevent the formation of aluminum oxide that degrades ultraviolet reflectivity. In some cases, original holographic gratings are used where the groove structure is formed in photo-active materials such as photoresist, however, this type of grating is difficult to overcoat with dielectric materials since it cannot withstand the high temperatures required to make a durable multilayer dielectric.
The distribution of incident field power of a given wavelength diffracted by a grating into the various spectral order depends on many parameters, including the power and polarization of the incident light, the angles of incidence and diffraction, the (complex) index of refraction of the metal (or glass or dielectric) of the grating, and the groove spacing. A complete treatment of grating efficiency requires the vector formalism of electromagnetic theory (i.e., Maxwell's equations), which has been studied in detail over the past few decades. The maximum efficiency of a grating is typically obtained with a simple smooth triangular groove profile, when the groove (or blaze) angle δ is such that the specular reflection angle for the angle of incidence is equal (in magnitude and opposite in sign) to the angle of diffraction. Ideally, the groove facet should be flat with smooth straight edges, and be generally free from irregularities on a scale comparable to the small fraction (< 1/10) of the wavelength of light being diffracted.
The distribution of power among the various diffraction orders depends on the shape of the individual grating grooves. Gratings are most efficient when light is specularly reflected into the diffracted beam, i.e., the grating appears to light up, or blaze, when viewed at the respective angle. This angle is called the blaze angle.
Grating efficiency typically falls off sharply away from the blaze angle in particular for high blaze angle gratings (Θ>40°), with strong dependence on the direction of polarization.
The diffraction efficiency of gratings operating near the blaze angle can be enhanced by applying metal (e.g., aluminum) and/or dielectric coatings. Such multi-dielectric coatings for improving grating diffraction efficiency of holographic gratings for tunable lasers were reported by D. Maystre et al. (Applied Optics, Vol. 19, No. 18, 15 Sep. 1980, p. 3099-3102). The efficiency with 2 stacks of two dielectric films increased from 77% to 87% for a wavelength of λ=0.64 μm.
Chandezon et al. (J. Opt. Soc. America, Vol. 72, No. 7, pages 839-846 (1982)) reported a rigorous formalism for computing the diffraction efficiency of multi-coated sinusoidal gratings. The gratings were coated with a stack of two dielectrics, with each dielectric layer having an optical thickness of λ/4 at normal incidence for a wavelength of λ=0.59 μm. The thicknesses of the dielectrics were chosen to produce a minimum of absorption, as for a mirror.
Multilayer dielectric diffraction gratings produced by depositing a coating on optically flat substrates were reported by M. D. Perry et al. (Optics Letters, Vol. 20, No. 8, Apr. 15, 1995, p. 940-942. These gratings have a high diffraction efficiency and a high optical damage threshold at a wavelength of λ=1053 nm.
The reported dielectric-coated gratings, however, already have a relatively high diffraction efficiency without the applied coating. The coating further enhances the diffraction efficiency, which can be expressed by an empirical rule stating that each additional stack of λ/4 coatings results in a decrease of the absorption by a factor of 2. Therefore, if a grating has a diffraction efficiency of 90%, an additional stack would boost the efficiency to 95%, a further stack to 97.5%, and so on. However, none of the references suggests that the diffraction efficiency of ruled gratings, such as symmetric triangular gratings, can be increased by, for example, an order of magnitude, at wavelengths where the efficiency of uncoated or metal-layer coated gratings is so low as to render the gratings useless.
It would therefore be desirable to provide an easily manufacturable grating structure that operates at a low diffraction order and has a high diffraction efficiency and dispersion, in particular for UV laser applications.